The brightness of the projection display device including an illumination optical system including a light source, a light valve (display device), and a projection optical system is mainly determined by the etendue of the light source, the brightness of the light source, the transmittance of each optical system, and the etendue of the projection optical system. When the etendue of the light source and the etendue of the projection optical system match each other, the efficiency of the entire projection display device is determined by the transmittance of each optical system. However, when the etendue of the light source and the etendue of the projection optical system do not match each other, losses occur due to a factor other than the transmittance of each optical system among the aforementioned factors that determine the brightness of the projection display device. Consequently, the efficiency of the entire projection display device is lower.
The etendue of the light source is determined by a product of the light-emitting area and the light-emitting angle of the light source. In other words, in the case of a surface-emitting light source such as a LED (light-emitting diode), the etendue ELight is expressed by the following formula (refer to Patent Document 1):ELight=πALight sin2(θLight)(ALight: light-emitting area of light source, θLight: maximum light-emitting angle of light source)
The etendue EMD of the projection optical system is determined by the size of the display unit of the light valve and the F-number of a projection lens as follows (refer to Patent Document 2):EMD=πAMD/4(F/#)2 (AMD: surface area of light valve, F/#: F-number of projection lens)
To prevent wasteful loss of the light emitted from the light source at the projection optical system, it is desired that the etendue ELight of the light source and the etendue EMD of the projection optical system match each other or the etendue EMD of the projection optical system be larger than the etendue ELight of the light source. This means that the light-emitting area or the light-emitting angle of the light source is desirably small.
As in the case of a configuration including a DMD (digital micromirror device) as the light valve, in a projection display device where there is no need to align polarization, it can be said that the etendue ELight of the light source and the etendue EMD of the projection optical system match each other at the time of ELight≦EMD.
On the other hand, as in the case of a configuration including a LCD panel (liquid crystal display panel) as the light valve, in a projection display device where it is necessary to align polarization, the etendue of the light source is effectively double, and thus it can be said that the etendue ELight of the light source and the etendue EMD of the projection optical system match each other at the time of ELight≦EMD.
A high-pressure mercury lamp is mainly used as the light source of the projection display device. However, a need has arisen for a light source that does not include any environmental pollutants such as mercury, and a LED and a laser light source are now focuses of attention as candidates for next generation light source.
The LED has characteristics in which etendue ELight of the light source is larger than that of a high pressure mercury lamp and in which the light emitting efficiency is lower than that of a high pressure mercury lamp because of a large light emitting area. Thus, to realize a projection display device equal in brightness to the high-pressure mercury lamp by using the LED, the etendue EMD of the projection optical system must be increased. For example, the size of the display unit of the light valve is increased. This consequently leads to enlargement of the entire device or to a cost increase.
On the other hand, the laser light source has characteristics in which etendue ELight of the light source is much smaller than that of a high pressure mercury lamp and in which light emitting efficiency is higher than that of a high pressure mercury lamp because of a very small light emitting area. Accordingly, when the laser light source is used, the etendue EMD of the projection optical system can be reduced more than when the high-pressure mercury lamp is used. Thus, the entire device can be miniaturized, and high efficiency and low costs can be achieved. However, severe restrictions such as safety standards are imposed on the laser light source, and all products including laser light sources cannot be freely made commercially available. Only products that comply with restrictions, such as safety standards, can be made commercially available. As a result, there is a limit on the brightness of the commercially available projection display device.
To overcome problems in the projection display device that includes such two types of next-generation light sources (LED and laser light source), a projection display device that includes a novel light source configured to use a laser as excitation light and to convert a wavelength and emit light by a phosphor has been developed. This novel light source has a light-emitting area that is smaller than that of the LED, and the etendue ELight of the light source is smaller than that of the LED but larger than that of the laser light source. While dependent on the efficiency of the laser as the excitation light, light-emitting efficiency is higher than that of the LED, particularly in the case of a green laser, and equal to that of the LED in the case of a red laser. Thus, when a configuration where a green light source is a laser light source for emitting light from a phosphor by using a laser as excitation light is employed, a projection display device that is smaller and brighter than a configuration, where the light sources of all three colors of red, green, and blue are LEDs, can be realized. Such a projection display device can achieve brightness equal to that of an existing projection display device including a mercury lamp when such a projection display device has almost same size as the existing projection display device. Further, when the light sources of all three colors of red, green, and blue are laser sources for emitting light from a phosphor by using a laser as excitation light, common use of one laser light source among the respective colors can reduce the number of laser light sources, and thus is effective for achieving lower costs.
The light output of the laser necessary in such a novel light source for converting a wavelength and emitting light at a phosphor using the laser as excitation light is several W to several tens of W. The spot size of a laser to be applied is very small (equal to or less than about 1 mm2) Consequently, in a configuration where a phosphor is applied on a glass substrate or an aluminum substrate to be fixed, the phosphor and a binder necessary for applying the phosphor are burned away by heat accompanying the laser irradiation. To deal with this problem, in Patent Documents 3 and 4, a phosphor color foil prepared by applying a phosphor on a disk-shaped glass substrate or aluminum substrate is used. When the laser is applied while rotating the phosphor color foil, the laser is not continuously applied only to the same phosphor particles but is applied to phosphor particles that are irradiated with the laser and that sequentially change accompanying the rotation of the color foil. As a result, even when the laser having a light output of several tens of W is applied to the phosphor, the phosphor can be used without any deterioration in its characteristics.